The major experiments and studies projected for 1975-76 concern the mode and extent of heteroduplex formation and correction (recombination) in both nuclear and mitochondrial DNA. For nuclear DNA we will assess the polarity of correction in (and failure of correction) several newly synthesized strains heteroallelic at certain loci and variously heterozygous elsewhere in the remaining genome. Allelic identification tests that preserve the geometric integrity of directly replica-plated ascosporal clones provide the basis for unambiguous and reliable detection of all conversions and postmeiotic segregations among unselected tetrads. Genetic factors modifying the extent of recombination over small regions will be evaluated in a series of six related hybrids that collectively mark and thereby provide recombination control over the total length of chromosome VI. These genetic studies will doubtless permit a decision on the question of whether heteroduplex nuclear DNA originates modally in a symmetrical or reciprocal manner or in an asymmetrical fashion generating only a single heteroduplex. Also, the effect of specific chemical agents on heteroduplex formation and correction will be investigated. Additionally, the genetic basis for polarity in gene conversion will be critically re-examined.